Several methods have been adopted to automatically adjust the focus in scanning electron microscopes. One of them is illustrated in FIG. 5.
As shown in FIG. 5, a deflection unit 14 receives a timing signal from a control unit 5. The deflection unit 14 creates a horizontal, or X-direction, scanning signal and a vertical, or Y-direction, scanning signal according to the timing signal, and supplies these signals to a deflection coil 2. The control unit 5 produces a digital signal for indicating the value of the exciting current fed to an objective lens 3. This digital signal is converted into analog form by a digital-to-analog (D/A) converter 15 and sent to a driver circuit 16. The electron beam 1 emitted by an electron gun (not shown) is deflected in the X- and Y-directions by the deflection coil 2. The beam 1 is focused by the objective lens 3, and a desired region on a specimen surface 4 is scanned with this focused beam 1. During this scan, the value of the exciting current supplied to the objective lens 3 is changed in increments of .DELTA.I within a given range. In particular, as shown in FIG. 6, whenever a given time T.sub.0 elapses, the exciting current is varied by .DELTA.I. During the given time T.sub.0, the electron beam 1 raster scans the specimen surface for obtaining one frame of image. The secondary electrons released from the specimen surface 4 during this scan are detected by a detector 6 and converted into an electrical signal, which is sent to a signal-detecting unit 7 having a low-pass filter 8 and a high-pass filter 9. These filters 8 and 9 remove noises from the incoming signal. The signal from which the noises have been removed is supplied to an absolute value circuit 10 which takes the absolute value of the input signal. An integrator circuit 12 integrates the signal passed through the absolute value circuit 10 which corresponds to one frame. The output signal from the integrator circuit 12 is converted into digital form by an analog-to-digital (A/D) converter 13 and fed to the control unit 5. It is assumed that when the exciting current fed to the objective lens assumes values I.sub.1, I.sub.2, I.sub.3, . . . I.sub.n, the integrated values supplied to the control unit 5 are S(I.sub.1), S(I.sub.2), S(I.sub.3), . . . S(I.sub.n), respectively. The control unit 5 calculates the relation between the exciting current and the integrated value from the above values, and then finds the objective lens-exciting current value I.sub.0 (FIG. 7) giving the maximum integrated value from the calculated relation. This current value I.sub.0 is taken as the exciting current which provides the focused condition. Therefore, the objective lens is set at this current value I.sub.0 to achieve focusing.
In the above-described prior art automatic focusing system, focusing is attained by evaluating the output signal from the secondary electron detector, the output signal being derived immediately after the objective lens current is varied in an increment as shown in FIG. 6. This makes it impossible to accomplish accurate focusing. Specifically, the exciting coil of the objective lens has a large inductance. Therefore, if the current is varied, the current flowing through the exciting coil of the objective lens does not vary instantaneously; rather it changes gradually as indicated by the dotted lines in FIG. 6. Accordingly, if one evaluates the output signal from the secondary electron detector and judges the focused condition immediately after the exciting current is varied as mentioned previously, the focused condition is not maintained when the exciting current settles into stationary state.
One contemplated method for solving this problem is to detect and evaluate the output signal from the secondary electron detector after the amplitude of the exciting current fed to the objective lens has settled into stationary state. Then, the exciting current providing focused condition is determined. However, it is time-consuming to perform automatic focusing action by this method.
Scanning electron microscopes free of this problem have been developed. In particular, a small auxiliary coil is disposed close to the objective lens. The exciting current supplied to this auxiliary coil is varied to search for the focus. One example of such scanning electron microscopes is disclosed in U.S. Pat. No. 4,199,681. In this disclosed microscope, if the exciting current to be supplied to the auxiliary coil for achieving focused condition is determined, then the focusing operation is completed while this current is kept supplied to the auxiliary coil. However, if an electrical current close to the maximum allowable current value has already flowed through the auxiliary coil when the focusing operation is completed, then it is not possible to widen the searched area in the direction to increase the current according to the changing position of the specimen. Eventually, it is only possible to search a narrow area for the focus. This type of electron microscope has another disadvantage. Specifically, if the electrical current flowing through the auxiliary coil is relatively large after the completion of the focusing action, a slight deviation of the axis of the auxiliary coil from that of the objective lens produces a distortion in the image. This makes it impossible to observe good-quality images.